1. Field of the Invention
The present invention relates to a tyre for a motor vehicle, in particular to a high-performance tyre.
2. Description of the Related Art
Motor vehicle tyres having a tread provided with blocks delimited by circumferential grooves extending in a substantially longitudinal direction and by transverse grooves extending in a substantially axial direction are known. The blocks resulting from the intersection of said grooves are formed in various suitably designed shapes and are arranged in adjacent circumferential rows, each of which is located between two successive circumferential grooves.
The circumferential grooves may influence the directional and travel stability properties of the tyre in relation to the lateral (slip) thrusts directed parallel to the rotation axis of the tyre.
The transverse grooves, in turn, may influence the traction properties of the tyre, namely its capacity to transmit efficiently to the road surface the tangential thrusts parallel to the direction of travel during acceleration and braking of the motor vehicle.
The circumferential grooves may also influence the draining of the water in the area making contact with the road surface-(footprint area) during travel on a wet road surface.
Furthermore, the presence of transverse and circumferential grooves influences the rolling noise of the tyre. In fact, one of the main causes of noisiness is the continuous succession of impacts of the edges of the blocks on the road-surface.
A further cause of noisiness consists in dragging of the blocks on the road surface when they enter and leave the area of contact with the road surface. This dragging is due essentially to the deformation of the tread both when the tyre is flattened against the road surface and when it recovers its inflated condition, as it leaves the area of contact with the road surface.
The deformations of the tread when coming into contact with and leaving the area of contact with the road surface also produce a cyclical volumetric variation of the grooves which delimit the blocks and a consequent cyclical action of compression and expansion of the air trapped inside the grooves. These phenomena of compression and expansion of the air increase the rolling noise of the tyre.
Various measures which tend to limit the rolling noise of a tyre are known. One of these consists in providing the blocks with different longitudinal dimensions by adopting two or more different-pitch values distributed in circumferential succession—called “pitch sequence” —such as to provide the maximum possible lack of uniformity over the circumferential extension of the tread. The aim is to distribute the acoustic energy due to the impacts and dragging of the blocks over a wide spectrum of frequencies, thus avoiding concentrating it in a specific frequency and producing bothersome noise.
The noisiness of a tyre, measured in accordance with New Dir. 2001/43/EC Aug. 4, 2001, is considered unacceptable when it exceeds the following limits:
tyre width 145 mm72 dB(A)tyre width >145 and 165 mm73 dB(A)tyre width >165 and 185 mm74 dB(A)tyre width >185 and 215 mm75 dB(A)tyre width >215 mm76 dB(A)
The noisiness of tyres is a problem which is difficult to solve because some measures which tend to reduce it adversely affect the directional, traction and water drainage properties.
EP 1 189 770 B1 describes a high performance tyre for a motor vehicle provided with a tread having an overall width L and comprising two deep circumferential grooves which separate a central region from two lateral shoulder regions, said shoulder regions being provided with shoulder blocks, the sum of the widths of said lateral shoulder regions being equal to or less than 60% of the overall width L, the width of each of the shoulder regions being not less than 20% of said overall width L, each of said circumferential grooves being adjacent, on the side further from the central region, to a continuous track from which are branched transverse grooves which delimit said shoulder blocks, said continuous track terminating in a continuous wall which forms a lateral wait of said circumferential groove, characterized in that said continuous lateral wall of at least one circumferential groove has a profile, in a radial plane, which is more inclined, with respect to a center-line axis of said circumferential groove, than the profile of the facing lateral wall of said circumferential groove, and in that the blocks of the central rows are separated by transverse grooves having a bottom wall with a shaped profile of variable depth.
EP 0 812 709 describes a tyre having a tread comprising two regions at least one of which is provided with a plurality of inclined main grooves, each of which comprises a segment with a steep inclination and a segment with a slight inclination. A lateral band of the two regions has an auxiliary groove with a steep inclination communicating with two adjacent, inclined, main grooves and an auxiliary groove with a slight inclination, situated between the two adjacent, inclined, main grooves. In a first embodiment, the inclined main grooves extend from a circumferential groove close to the equatorial plane of the tyre, while in a second embodiment they have a blind bottom. In a third embodiment, the steeply inclined segments of two adjacent, inclined, main grooves are joined by a thin transverse groove.
EP 0 867 310 describes a tyre comprising blocks formed in a tread portion by means of a plurality of circumferential grooves and a plurality of directionally inclined grooves. At least some of the directionally inclined grooves extend from a circumferential groove close to the equatorial plane of the tyre and extend towards one end of the area of the tread making contact with the ground. Each, of the blocks has an angled portion which forms an acute angle of 10°-60 defined by a circumferential groove and a directionally inclined groove. The surface of the angled portion of the block is chamfered over a distance of 10-30 mm from a tapered end thereof in a longitudinal direction so as to vary gradually towards, a portion of larger width.
Nowadays high performance vehicles more and by electronic systems. Such systems manage different driving conditions and modify the dynamic behavior of the vehicle for improving, for instance, stability, braking and acceleration performances thereof. In this scenario, high performance tyres are required to work in a different way with respect to tyres of the past and need to be redesigned (in terms of tread pattern and internal, structure thereof) so as to meet the relevant changes introduced in the vehicle behavior by the electronic systems.
It can be pointed out that high performance vehicles have a negative camber while running under normal driving conditions, but have null or positive camber when high lateral accelerations and/or quick load transfers, occur. Since the electronic systems sensibly and actively intervene on the vehicle; behavior, the tread pattern designs have, to take into account said fact.
In the following description, the terms “HP” (High Performance) or “UHP” (Ultra High Performance) tyres indicate tyres which are suitable for applications involving high operating speeds (e.g. higher than 200 km/h) and/or extreme driving conditions. In particular the terms High Performance or Ultra High Performance tyres are intended to indicate tyres which belong to Classes “H” and “V” (maximum speed over 210 Km/h) and to Classes “W” and “Y” (maximum speed over 240 Km/h).